If two dwelling units are supplied by a single feeder or service, and the calculated load under Part III (Standard Method) exceeds that for three identical units calculated under 120.84, the lesser of the two loads may be used. This method provides a code-compliant and efficient approach to sizing the service or feeder for the entire building.
So in accordance with NEC 120.85, the calculation starts with the load of the larger unit, considered as the “third unit” for demand factor purposes.The demand factor for a three-unit dwelling (45%) is applied to estimate the total service load for a duplex.
Code editionNEC 2023NEC 2026 New
NEC 2026: load calculations are in Article 120 (formerly Article 220). Use it only where the 2026 NEC is adopted.
Duplex Home Load Calculation
Unit __number__ Details
Dwelling Unit
2 if both dwelling units are identical; otherwise 1 here and add a second unit type.
Per dwelling unit, from outside dimensions (NEC 120.5(C)).
1,500 VA each, minimum 2 (120.82(B)(2)).
1,500 VA each, minimum 1.
Fixed Appliances
Nameplate VA. Enter 0 if not installed.
Other Appliances
Pool pump, spa, sauna, etc. Nameplate VA.
Cooking, Laundry & Water Heating
Nameplate VA (kW × 1000). Enter 0 if not electric.
Do not use Table 120.55 values.
California50A, 240/208V labeled circuit for future range (2025 Energy Code).
California30A, 240/208V labeled circuit for future dryer (2025 Energy Code).
California30A, 240/208V labeled circuit for future water heater (2025 Energy Code).
Heating & Cooling
The larger of the heating and cooling loads is used (120.82(C)).
Condensing unit or heat pump. 0 if none.
Fan motor only (excludes supplemental heat).
Total nameplate kW.
4 or more: 40%. Fewer: 65%.
EV Charging
Counted at 100% (no demand factor).
Per dwelling unit. Use charger nameplate if installed. 0 if none.